sand-dune (n.)

"water-worn detritus finer than gravel; fine particles of rocks (largely crystalline rocks, especially quartz); the material of the beach, desert, or sea-bed;" Old English sand, from Proto-Germanic *sandam (source also of Old Norse sandr, Old Frisian sond, Middle Dutch sant, Dutch zand, German Sand), from PIE *bhs-amadho- (source also of Greek psammos "sand;" Latin sabulum "coarse sand," which is the source of Italian sabbia, French sable), suffixed form of root *bhes- "to rub."

Historically, the line between sand and gravel cannot be distinctly drawn. Used figuratively in Old English in reference to innumerability and instability. General Germanic, but not attested in Gothic, which used in this sense malma, related to Old High German melm "dust," the first element of the Swedish city name Malmö (the second element meaning "island"), and to Latin molere "to grind."

Metaphoric for innumerability since Old English. In compounds, often indicating "of the shore, found on sandy beaches." In old U.S. colloquial use, "grit, endurance, pluck" (1867), especially in have sand in (one's) craw. Sands "tract or region composed of sand," is by mid-15c.

"mound, ridge, or hill of loose sand heaped up by the wind near the coast of a sea," 1790, from French, Middle Dutch or Middle Low German dune, all of which are perhaps from Gaulish *dunom (making it cousin to down (n.2) "small, rounded hill").

The French dune "sand hill" (13c.) is held by Diez to be an Old French borrowing from Dutch duin or some other Germanic source. Italian and Spanish duna are from French. The English word is perhaps also partial a dialectal form of down (n.2). Dune buggy, "recreational motor vehicle designed for use on beaches," is attested by 1965.